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Hydrolysis of ß-Casein and Peptides by Intracellular Neutral Protease of Streptococcus diacetylactis

Laboratoire de Biochimie et Technologie Laitières and Laboratoire de Microbiologie Laitière et de Génie Alimentaire, Institut National de la Recherche Agronomique, 78350 Jouy-en-Josas, France

ABSTRACT

The endopeptidase activity of mesophilic streptococci was characterized further by investigating the specificity of an intracellular endopeptidase from Streptococcus diacetylactis for ß-casein, derived peptides, and bradykinin. The inhibitory action of phosphoramidon as well as direct determinations of metal content showed this enzyme was a metalloprotein. Hydrolysis of native ß-casein was relatively low. Peptides obtained from the fraction soluble at pH 4.6 led to the demonstration that Pro₁₈₆-Ile₁₈₇ and Ala₁₈₉-Phe₁₉₀ were hydrolyzed by the enzyme. Two peptides derived from ß-casein by the action of chymosin were hydrolyzed efficiently: we observed hydrolysis of Lys₁₇₆-Ala₁₇₇, Lys₁₆₉-Val₁₇₀, and Pro₂₀₆-Ile₂₀₇. The Pro₇-Phe₈ bond of bradykinin was hydrolyzed rapidly, showing that this enzyme was efficient for the hydrolysis of prolyl peptide bonds. The protease was slightly less sensitive to phosphoramidon than was thermolysin. Metal analyses showed the enzyme contained 580 µg of zinc and 4,760 µg of calcium per gram protein. This protease is thus a truemetalloenzyme (E.C.3.4.24.4), and its action may complete the hydrolysis initiated by chymosin remaining active in cheese curd by hydrolyzing peptides released by chymosin.